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M4. Chiswick to Slough By-pass (J1 to J5)

The Chiswick Flyover

The Chiswick Flyover was the first major two-level highway scheme to be carried out in the Metropolitan Area since World War II. The works extended for about half a mile and included a through road to link London's "new" western approach, the Cromwell Road Extension, to the Great West Road without interference from cross-traffic. This through road was carried over a new 400ft diameter roundabout with a 40ft wide carriageway at the junction of the North Circular Road, Chiswick High Road, the road to Kew Bridge and the Great West Road. Traffic was distributed to these roads by means of four slip roads joining the roundabout at ground level. The junction was used by at least 40,000 vehicles a day and was seriously congested at times. The scheme removed a potential bottleneck at the western end of the Cromwell Road Extension.

The through road of the flyover had dual 24ft 0in wide carriageways separated by a central island and each slip road was 24ft wide, providing two traffic lanes. The gradient of each slip road east of the roundabout was about 1 in 20 and that of the through road west of the roundabout about 1 in 29. Other gradients did not exceed 1 in 40.

The superstructure of each span consisted of fifteen concrete beams placed side by side, each beam being precast in three sections. The end sections were about 38ft long and the centre sections 48ft long, each section weighing about 33 tons. The beam units were of inverted `T' section with a narrow top flange and with transverse diaphragms cast integral with the unit. The bottom flange of each roadway unit contains eighteen 1¾ in diameter straight ducts formed by casting in steel sheathing. The outer beams of each span were of `U' section constructed of concrete with grey Cornish granite aggregates, the exposed vertical face being lightly bush hammered.

The sections were manufactured in a factory some 60 miles away and transported daily to the site where they were lifted by means of a 45 ton Lima crane and erected directly on the permanent bearings on piers and abutments and on temporary rollers supported on steel trestles on either side of the transverse joints between the sections. Each complete beam after assembly was post-tensioned by the Freyssinet system. An in-situ reinforced concrete slab was cast at the level of the top of the beams and the deck prestressed transversely by the Gifford-Udall system with cables threaded through the webs and transverse diaphragms of the beams. The anchorages were located in the inner webs of the outer `U' beams.

A loading test to failure of one complete roadway beam was carried out in order to verify assumptions made in the design, both in the elastic and ultimate conditions, and to provide information for research on prestressed concrete. The test rig was designed to be capable of applying symmetrical loads of 125 tons at two points 30ft apart. The beam failed when the applied loads reached the value of 118 tons on each jack, in close agreement with the estimated failing load. The actual failing load represents a load factor of three on the combined dead and live loading in the design.

Fixed and rocker bearings were provided alternatively on abutments and piers. The bearing plates and rockers were made of Meehanite `CB' metal, a high tensile corrosion-resisting cast iron, and the locating pins were of stainless steel. Expansion joints in the carriageways and verges above the "free end" bearings consisted of stainless steel sliding plates.

The Wellesley Road Bridge bridge was constructed in two halves separated by a longitudinal joint. The abutments were of brick faced mass concrete on reinforced concrete raft foundations. The complete deck consisted of 43 precast prestressed beams of inverted `T' section post-tensioned during manufacture by the Freyssinet system. The beams were 72ft long, each weighing about 24 tons. They were placed side by side on the bearings and in-situ concrete transverse diaphragms cast between the beams and around metal ducts passing through the beam webs. The joints between the beam flanges were filled with concrete and an in-situ concrete deck cast at the level of the top of the beams. Each half of the deck was pre-stressed transversely by the Freyssinet system.

Rubber bearings were adopted for this bridge. The outer edge beams were constructed of concrete with grey Cornish granite aggregates, lightly bush hammered on the exposed face.

The scheme opened to traffic on September 30th 1959. The consulting engineers to the Ministry of Transport and Civil Aviation for the design and supervision of the Chiswick Flyover Scheme were Messrs. Harry Brompton & Partners and the consulting architect was George Stewart. The main contractors for the flyover were Alderton Construction Co. Ltd. The precast beam units were manufactured by the Cowley Concrete Co. Ltd., Meehanite bridge bearings by the Butterley Co. Ltd. and the rubber bearings by Andre Rubber Co. Ltd. The sub-contractors for the bridge piling were Braithwaite Foundations and Construction Ltd.

The flyover was constructed through a densely built-up area but the demolition of buildings was kept to a minimum consistent with engineering requirements. The scheme also involved the diversion of many miles of underground services in the shape of G.P.O. cables, gas mains, sewers, etc.

The Contractor's programme was governed to a large extent by the requirement that construction operations should not impede the smooth flow of traffic, and the work was divided into stages, necessitating careful planning. A temporary slip road on the south side of the works connecting the Cromwell Road Extension to Chiswick High Road together with a temporary enlargement of the original small roundabout were constructed first in order to provide additional capacity for increasing traffic flow resulting from completion of sections of the Cromwell Road Extension. Traffic was diverted from this temporary road to the completed northern slip roads and the northern arm of the new roundabout to enable construction on the south side and further bridge work to proceed.

The flyover embodied two bridges:

the main bridge consisted of four freely supported square spans each of 125ft, with a width of 59ft between parapets, carrying the through road over the new roundabout and

a bridge over Wellesley Road, east of the new roundabout, 70ft single skew span and 124ft between parapets, as the slip roads met the through route at this point and four 24ft carriageways were required.

The approaches of the flyover and the section between the bridges were formed on embankments retained between mass concrete walls, brick faced, with in-situ concrete copings forming guard verges about 4ft wide to the through road and slip road carriageways. The embankment filling consisted of well graded mechanically stable gravel (hoggin) containing between 5 per cent and 15 per cent by weight of material passing a No. 200 sieve. The filling was spread in 9in layers and each layer compacted by nine passes of 8 ton smooth wheel rollers, a state of compaction corresponding to about 71 per cent air voids. Against bridge abutments and retaining walls, compaction was carried out by means of vibrating plate compactors, the degree of consolidation behind bridge abutments corresponding to about 5 per cent air voids.

Carriageway construction consisted generally of a reinforced concrete base slab 10in thick with a surfacing of 3½ in of hot rolled asphalt. Curing of the concrete was carried out by bituminous emulsion spray. Transverse joints were formed in the base slab at a maximum spacing of 120ft and a strip of light steel fabric was laid flat on the base slab at the joints before surfacing, as a precaution against possible cracking of the asphalt.

All structures of the flyover were designed for the Ministry of Transport Standard Highway Loading and for the 180 ton abnormal live load in accordance with British Standard 153 (Part 3A) 1954, Section A.

The abutments and the two easterly piers of the main bridge were supported on reinforced concrete rafts founded on a stratum of compact gravel overlying the London Clay. The western pier was carried on a narrow piled foundation owing to the proximity of an existing 48in diameter sewer at a depth of 20ft below ground level. The bridge abutments were of mass concrete, brick faced, and each pier consists of five massive concrete columns, lightly reinforced, and a heavily reinforced cap beam.